In oil and gas well drilling operations, if sand enters the production pipe or the area between the production pipe and the casing, a number of problems can follow. These problems include production loss caused by sand bridging in casing, tubing and/or flow lines; failure of casing or liners due to the removal of surrounding formation, compaction and erosion; abrasion of downhole and surface equipment; and the need to dispose of unconsolidated materials from the recovered hydrocarbons.
The sand usually comes from unconsolidated formations. Its entry can be controlled through chemical or mechanical means to prevent the occurrence of the foregoing problems. One mechanical means for preventing sand influx is the gravel pack.
Gravel packing is a method of forming a filter of gravel between the producing formation and the production pipe. In a cased well which has been completed by perforation, the gravel pack is normally situated between the production pipe and the casing. If used in an uncased or open hole, the gravel pack serves both as a filter and also to support the unconsolidated formation. It also assists in supporting the formation in a cased hole, though the support is less important as the casing serves most of the supporting function.
A gravel pack is formed around the production pipe and disposed adjacent the producing formation. A liner or screen, having a plurality of narrow, spaced-apart slots or screen-covered openings through which the formation fluids enter the production pipe from the formation, is attached around the production pipe. The screen surrounds a body of gravel which serves as a filter to screen out fine sand and other unconsolidated products as the well fluid flows from the formation into the production pipe. Fluids thus enter the production pipe relatively free of the sand or unconsolidated material from the producing formation. The gravel which surrounds the screen should be packed to sufficient height and volume to remain consolidated and not be displaced as it filters the in-flowing well fluid.
Isolating the gravel pack ensures that produced fluid does not flow between the production pipe and the casing (or between the production pipe and the well bore in an uncased well). Isolation is accomplished in part by placing a packer above the gravel pack. The packer seals in the area between the casing (or well bore) and the production pipe. The area below the gravel pack can be sealed to complete isolation either by locating another packer (preferably a non-releasable "sump packer") below the gravel pack, or by providing a bridge plug below the gravel pack. Once the gravel pack is isolated, the produced fluid, which is under pressure, is forced to flow through the gravel before entering the production pipe.
In the past, compression-set packers with lockdown features have been used to seal against the inside of the well casing or well bore. In such packers, slips are mechanically actuated to anchor the packer to the casing wall (or to the uncased well bore). The sealing elements, typically made of rubber, are then energized by compressing them between two shoulders disposed on the packer. One shoulder is typically provided by a shoe below the rubber sealing elements, and the other by one or more mandrels passing through the inside of the rubber sealing elements. A lock ring and ratchet system is often used to prevent the shoulders from slipping away from the seal energizing position.
It has been found that compression-set packers will leak at high pressures unless they include a means for increasing the seal energization, such as a pressure responsive self-energizing feature. Leakage occurs because even when a high setting force is used to set the packer seals, once the setting force is removed, the ratchet will retreat slightly before being arrested by the locking effect created when the sets of ratchet teeth mate firmly at the respective bases and apexes of each. This causes a loosening of the seal. Packers are also particularly prone to leak if fluid pressures on the packers are cycled from one direction to the other.
In a typical conventional packer used in the past, an increased energizing force will be applied to the rubber sealing elements from the mandrel or mandrels when fluid pressure is applied to the packer from one direction, but not when pressure is applied from the other direction. In cases where one mandrel is employed, the cross-sectional area of the mandrel is usually exposed to the pressure differential and creates an energizing force against the rubber sealing elements in one direction only. Leakage occurs when pressure is applied in the opposite direction, in which case there is typically no energizing force against the sealing elements resulting from pressure on the cross-sectional area of the mandrel. Where two or more mandrels are employed, in conventional packers there typically are seals between the mandrels, and the pressure differential acting on the combined cross-sectional areas of the mandrels provides the self-energizing force in the one direction, but again, typically not in the other direction.
There have been several suggested solutions in the past to the general problem of pressure-deactivation of well packers. Each of these proposed solutions attempts to increase the seal energizing force when fluid pressure is applied, in some cases from annulus pressure above or below the packer, and in at least one case from pressure applied through the central bore of the inner mandrel. An example of the former type of system is disclosed in U.S. Pat. No. 4,224,987, issued Sept. 30, 1980, to Allen. Allen discloses a well packer using a combination of an upper movable shoe and sleeve, and possibly some inner mandrel movement, to increase seal element energization from annulus pressure applied from above, and a movable piston to increase seal element energization from annulus pressure applied from below. An upper shoe and sleeve are slidably retained on the inner mandrel in engagement with the seal elements, and are responsive to fluid pressure applied from above. The upper shoe and sleeve move down in response to such pressure, further compressing the packer elements. From below, annulus pressure acts upwardly on a telescoping piston, forcing it further into engagement with the packer seals. Thus, the Allen device utilizes movable shoes/pistons both above and below the seal elements, and requires a plurality of moving sleeves, pistons, and other parts both above and below the seal elements in order to effect the disclosed self-energizing of the seals. Accordingly, the Allen device is unduly complicated and difficult to make and use, and, with its multiplicity of moving parts, more likely to experience malfunctions than simpler packers. Moreover, the Allen seal elements are actuated in such a way that the movable sleeves/pistons which effect the increased energization engage the seal elements across only a part of their diameters, causing extrusion of the elastomeric members around them at the upper and lower extremities of the stack of seal elements. Such extrusion around the sleeves and pistons can cause uneven stresses in or even damage to the seal elements, and could lead to seal failure. The Allen device does not simply divide up the mandrel cross-sectional area(s) to respond to fluid pressure differentials acting from both above and below the packer to increase seal energization, as does the present invention.
Another approach to self-energization of a well packer due to pressure applied from both above and below the packer is disclosed in U.S. Pat. No. 3,459,261, issued Aug. 5, 1969, to Cochran. The Cochran device discloses a floating sleeve on which the seal element is mounted, the floating sleeve being slidable between abutments and responsive to fluid pressure applied from above and below the packer to increase the endwise compression of the seal. Like the Allen device, the Cochran packer thus has movable shoes/sleeves both above and below the seal element, and is similarly unduly complicated. Moreover, since the sliding sleeve of Cochran must remain free to move alternately up and down in order to effect self-energizing in the event of pressure cycling, this increases the chances of a failure to self-energize in at least one direction, in the event, for example, that the sleeve were to become stuck or otherwise prevented from moving fully or properly in one direction or the other. As in the case of Allen, the Cochran patent does not disclose simply dividing up the mandrel cross-sectional area(s) to respond to fluid pressure differentials acting from both above and below the packer to increase seal energization, as does the present invention.
Another approach to increasing seal energization is disclosed in U.S. Pat. No. 4,423,777, issued Jan. 3, 1984, to Mullins et al. The Mullins patent discloses a pressure chamber within a packer with dual-acting pistons, one piston setting the slips and the other piston compressing the seal elements. In the event that the seal elements begin to loosen, for example through extrusion, the Mullins patent discloses pressuring up through the central bore of the tool to the pressure chamber therewithin, thereby forcing the upper piston further into engagement with the seal elements and increasing the energization thereof. The increased energization therefore does not result from annulus pressure alone, as does the increased energization in the present invention. The Mullins device is therefore not fully self-energizing. In addition, if the Mullins internal pressure chamber should leak or otherwise fail, there could be no further energizing of the seals in the event of loosening through extrusion or the like. As in the case of Allen and Cochran, the Mullins patent does not disclose simply dividing up the mandrel cross-sectional area(s) to respond to fluid pressure differentials acting from both above and below the packer to increase seal energization, as does the present invention.
The present invention accomplishes self-energization of a well packer having a locking feature by simply dividing up the total cross-sectional areas of the mandrels into two components, one component creating a reduced self-energizing force when pressure is applied in one direction, and another component also creating a self-energizing force when pressure is applied from the other direction. This is accomplished simply by selectively locating and sizing two sets of seals on a plurality of telescoping members, without the need for movable pistons on each side of the seal elements, or a floating sleeve within the seal elements, or a pressure chamber within the packer with dual-acting pistons or the like, as in the case of the patents discussed briefly above.